1. Field of the Invention
This invention relates to an improved ultrasonic scanning system used primarily for scanning human tissue to determine its internal structure. A typical ultrasonic scanning unit was a transducer unit for sending and receiving ultrasonic energy. In a transmit mode of operation, the transducer sends incident ultrasonic energy into the patient's body. Variations in tissue density within the patient cause a portion of the ultrasonic energy to be reflected back to the transducer, where the reflected ultrasonic energy is converted into electrical signals. By the use of interpretative imaging circuitry and display apparatus, these electrical signals can be used to display to a diagnostician internal structure within the human body.
The use of multiple segment ultrasonic transducer units with accompanying actuation circuitry enables the examiner to steer and focus the ultrasonic energy to obtain high resolution "pictures" of the internal structure of the human body as that body is being scanned by the steering of the beam. In one such arrangement, a linear array of transducer energy conversion segments comprises the transducer unit. It is possible to selectively phase the actuation of these elements in this array in a known fashion during the transmit mode, according to a predetermined relative sequence, thereby producing a resultant incident ultrasonic beam which can be steered at a given angle with respect to the transducer unit emitting face and which can also be focused at a given fixed distance from that face.
When return echoes are received by an ultrasonic transducer, return focusing circuitry causes the transducer to focus the "listening" or reception portion of operation by selectively enhancing response to signals emanating from certain zones within the subject. This selective listening is achieved by delaying certain return signals relative to each other and then summing those return signals to produce an electronic signal which can be used to produce a visual display on a cathode ray tube or other viewing device. A technique of selectively focusing the return beam is known as "dynamic focusing". In dynamic focusing the zone of enhanced reception is caused to recede coincident with transmission of the incident ultrasonic energy through the patient.
The sending of multiple signals from a multiple element array presents problems with regard to waveform interference within the subject. A point source corresponding to one transducer element produces no interference effects, but a multiple element transducer surface arranged in a linear array approximates a multi-point source of wave energy with accompanying interference characteristics. Multiple point sources of wave energy tend to produce grating lobes or areas of maximum concentration as they propagate away from those sources. In the ultrasonic scanning application, the so-called zero order, or central, lobe is the area of concentration used to scan the subject.
Higher order maxima, or grating lobes, of the incident ultrasonic energy (generated when a resultant beam is produced by a phased transducer segment array) produce interfering spurious return signals. The electronic circuitry of ultrasonic systems is designed to process echo or return signals from the zero order or central lobe, where incident energy is concentrated. Signals returning from the grating lobes therefore create problems in imaging since the circuitry process those return signals as if they were coming from the zero order lobe or area of concentration. It is therefore apparent that this interference from grating lobe echoes degrades the final image quality.
2. Prior Art
One proposal for diminishing the adverse effects of grating lobe interference involves symmetrically reducing transducer element energy propagation about a midpoint in the transducer face. The resulting decrease in grating lobe interference is achieved, however, at the expense of decreasing the scanning resolution capability of the main lobe.
A second proposal reduces grating lobe interference while maintaining uniform energy propagation along the transducer surface. The second proposal achieves the reduction in side lobe interference by a reduction in the transducer element spacing. It is known, for example, that grating lobe interference can not only be reduced, but eliminated, if the transducer elements are spaced closely enough together. All grating lobe interference is eliminated when the spacing between adjacent transducer element midpoints is equal to or less than one half of the wave-length of the emitted sound energy.
Reduction of transducer element spacing, (with consequent increase in the number of elements per unit length) results in substantial increase in the quantity and cost of supporting electronics needed to coordinate both the transducer element firing and the sensing and interpretation of return signals. In a dynamic focusing and beam steering system, this increase is particularly significant, since the receiving electronics in such systems is quite expensive.
In processing return signals, system electronics must be provided to created delays for both (1) beam steering and (2) dynamic focusing. The beam steering delays are linearly related and are analogous to the beam steering delays described in reference to the transmit mode of system operation. They are constant for a particular direction of beam travel and require less sophisticated electronics than are required for producing dynamic focusing delays, which are time-varying and approximately quadratically related. It is thus the electronics necessary to produce dynamic focusing delays, which most significantly increases the system cost when attempting to eliminate grating lobe effects.
Complications are also encountered due in large measure to the fact that the dynamic focusing delays vary with the changing locations of the reception focal zones, i.e., must vary with time. Thus if the echoes are returning from an area fairly close to the transducer within the body the dynamic focusing delays will be relatively short. As the burst recedes deeper into the body the dynamic focusing delays will increase until they reach a maximum delay time.
One technique for providing a time varying dynamic focusing delay involves the utilization of charge coupled devices (CCD's). CCD's are expensive, and each transducer element requires its own charge coupled device and electronic support circuitry to dynamically focus the returning information.
This one-to-one matching of transducer elements with charge coupled devices and supporting circuitry has aggravated the increased cost and complexity required to eliminate the grating lobe interference problem.